In the art of making paper with modern high-speed machines, sheet properties must be continually monitored and controlled to assure sheet quality and to minimize the amount of finished product that is rejected when there is an upset in the manufacturing process. The sheet variables that are most often measured include basis weight, moisture content, and caliper (i.e., thickness) of the sheets at various stages in the manufacturing process. These process variables are typically controlled by, for example, adjusting the feedstock supply rate at the beginning of the process, regulating the amount of steam applied to the paper near the middle of the process, or varying the nip pressure between calendaring rollers at the end of the process. Papermaking devices well known in the art are described, for example, in "Handbook for Pulp & Paper Technologists" 2nd ed., G. A. Smook, 1992, Angus Wilde Publications, Inc., and "Pulp and Paper Manufacture" Vol III (Papermaking and Paperboard Making), R. MacDonald, ed. 1970, McGraw Hill. Sheetmaking systems are further described, for example, in U.S. Pat. Nos. 5,539,634, 5,022,966 4,982,334, 4,786,817, and 4,767,935.
In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (stock) on a traveling mesh wire or fabric and water drains by gravity and vacuum suction through the fabric. The web is then transferred to the pressing section where more water is removed by dry felt and pressure. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The papermaking machine is essentially a de-watering, i.e., water removal, system. In the sheetmaking art, the term machine direction (MD) refers to the direction that the sheet material travels during the manufacturing process, while the term cross direction (CD) refers to the direction across the width of the sheet which is perpendicular to the machine direction.
A wide range of chemicals is utilized in the papermaking stock furnish to impart or enhance specific sheet properties or to serve other necessary purposes. Such additives as alum, sizing agents, mineral fillers, starches and dyes are commonly used. Chemicals for control purposes such as drainage aids, defoamers, retention aids, pitch dispersants, slimicides, and corrosion inhibitors are added as required. Fabrication of quality paper required addition of the proper amount of these chemicals.
Wet end chemistry deals with all the interactions between furnish materials and the chemical/physical processes occurring at the wet end of the papermaking machine. The major interactions at the molecular and colloidal level are surface charge, flocculation, coagulation, hydrolysis, time-dependent chemical reactions and microbiological activity. These interactions are fundamental to the papermaking process. For example, to achieve effective retention, drainage, sheet formation, and sheet properties, it is necessary that the filler particles, fiber fines, size and starch be flocculated and/or adsorbed onto the large fibers with minimal flocculation between the large fibers themselves.
There are three major groups involved in wet-end chemistry: solids, colloids and solubles. Most attention is focused on the solids and their retention. In order to maximize retention, it is important to cause the fines and fillers to approach each other and form bonds or aggregates which are stable to the shear forces encountered in the paper machine headbox and approach system. In modern papermaking, this is usually accomplished by using synthetic polymers.
Control of wet-end chemistry is vital to ensure that a uniform paper product is manufactured. If the system is allowed to get out of balance (e.g., by over-use of cationic polymers), the fibers themselves will become flocculated and sheet formation will suffer. Also, functional additives (e.g., sizes, wet-strength agents) are often added at the wet end; if the chemistry is not under control, the functionality may not be adequately imparted and the product will be off-quality.
The wet end of a papermaking machine is critical in determining the longterm stability of the machine and ultimately the quality of the resulting product. Fluctuations in the volumetric flow from the headbox and/or in the composition (e.g., solids, fines and chemicals) of the pulp slurry or paper stock leaving the headbox will affect the percent solids of the wet fiber mat delivered to the dryer sections. These changes will be detected by the reel scanner but, because of the transport time delay and scanner response time, the dry end moisture control system is inadequate to compensate for these load disturbances.
During normal steady state operation of a papermaking machine, an equilibrium condition develops in the material balance of the short and long stock circulation loops. For the short circulation loop this means that the fines and filler retention of the paper web are in equilibrium with the concentration of these materials in the white water circulation; and for the long circulation this means that the fiber save-all operation, broke filler concentration, retention chemical concentration and furnish composition are stable.
Conventional methods for controlling the basis weight of the paper produced include regulating the paper stock flow rate from the stuff box through a basis weight or thick stock valve into the headbox. The valve is actuated in response to scanning sensor measurements of the paper just before the reel. The ability of this feed-back control technique to smooth out disturbances however is limited due to the long time lags through the machine from the thick stock valve to the reel.